1. Field
The present disclosure generally relates to the fabrication of composite laminate structures, and deals more particularly with automated placement of prepreg tows in high angle transition regions of a structure.
2. Background
Numeric computer controlled advanced fiber placement (AFP) machines may be used to layup large-scale, complex-shaped composite laminate structures. For example, in the aircraft industry, AFP machines may be used to layup composite airframe components such as spars and stringers. These AFP machines typically have one or more material placement heads that may be manipulated to apply and compact multiple prepreg tows on a mandrel or similar tool. Each tow comprises a formed tow prepreg or narrow strip cut from unidirectional tape which includes a bundle of fibers pre-impregnated with resin. In order to form nonlinear features or details of a structure, the material placement head is sometimes programmed to follow constant radius paths.
Automated layup of composite structures as discussed above can be challenging where the structure is highly contoured or has sharp geometrical features. For example, limitations on machine programming and/or material placement head movement may prevent layup of material around relatively sharp corners or highly angled bends, hereinafter referred to as “high angle transition regions” or “transition regions”. Material characteristics such as, without limitation, material width, tack and fiber stiffness may also limit material placement in high angle transition regions. Another problem that may be encountered when laying tows in high angle transition regions is wrinkling, buckling and/or distortion of the tows. Steering the tows around sharp, constant radius paths in these transition regions causes the inside radius of the tows to be placed in compression. This inside radius compression may force the fibers of the tow to gather, resulting in wrinkles, buckles and/or fiber distortion that may have an undesired effect on the mechanical performance of the structure.
Tow gathering in high angle transition regions may be reduced to some degree by using narrower tows, however the use of narrower tows reduces the rate at which material can be laid, thus reducing production efficiency, and may not be practical in some applications. Employing narrow tows may require the use of compensating reinforcements such as additional plies because narrow tows may cause undesired knockdown in some mechanical properties of a structure. These compensating reinforcements add undesired weight to the structure and may increase manufacturing costs.
In order to overcome the problem of material gathering when wider tows are used, and/or limitations on the movement of material placement heads, composite laminate structures having high angle transition regions are currently produced using a multi-step process in which a joint containing the high angle transition region is separately fabricated and then joined to straight sections of the structure. This solution to the problem is time-consuming, labor intensive and requires multiple, complex and expensive tools. Moreover, use of a separately fabricated joint may require the use of additional reinforcements in order to achieve structural performance requirements.
Accordingly, there is a need for a method of placing composite material in high angle transition regions which permits formation of complex geometrical features or details of a structure and which reduces or eliminates material wrinkling, bucking and/or fiber distortion, while permitting fabrication of the structure as a single component. There is also a need for a method of automated placement of prepreg tows within high angle regions and sharp corners or highly angled bends that is not limited by AFP machine programming capabilities and/or AFP material application head movements. Further, there is a need for a method of fabricating composite laminate structures having high angle transition regions that obviates the need for separately fabricated joints and multiple tools.